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and their mammalian relatives higher in the series; but it is
not a character that should have been made use of by Huxley,
since he believed in the existence of a corresponding element in
the Dog. As to the corpus callosum (Fig. 50, p. 77) being small,
that seems to be not more than a slight difference of degree.1
A number of other characters of secondary importance were added
by Huxley to the weight of evidence which led him to form a
group Metatheria for the Marsupials. Some of these, however,
are now known to be not evidence in that direction. For in-
stance he observed that no
Marsupial had more than a
single successional tooth. It
seems at the present moment
to be fairly clear that Marsu-
pials have a milk dentition
like other Eutherians, but
that only one of these teeth,
the fourth premolar, comes
to functional maturity. That
it is really one of a complete

milk   Series   is   evidenced   by"   FIG. 57.—Brain of Echidna aculeata; sagittal

the fact that this tooth is
lifferentiated contemporane-
3usly with another series
formerly held to belong to
bhe so-called prelacteal denti-

section., Anterior commissure ;
cbl, cerebellum ; c.mam, corpus mammil-
lare ; col.forn^ column of the fornix ; c.qut
corpora quadrigemma ; gang.hab, ganglion
linbenulare ;, hippocampal com-
missure ; med, medulla oblongata ;,
middle commissure ; olf, olfactory lobe ;
°pt> optic chiasma ; tiib.oJf, tnbereulum
olfactorium ; t?C7it. 3, third ventricle. (From
Parker and. Haswell's Zoology.}

.2 There still remains, of
course, the actual fact that
;he milk dentition is not for the most part functional, but its
significance breaks down with these fresh discovezies. Of this
Professor Osborn has remarked : " The discovery of the complete
louble series seems to have removed the last straw from the
heory of the marsupial ancestry of the Plaeentals," But Huxley
lid not lay much stress upon this matter of the teeth, since he
>bserved that similar suppressions of the milk dentition were to
>e found in many other mammals admittedly Eutherian.
Huxley regarded the peculiarities in the reproductive organs
1 Moreover, the " corpus callosum and the anterior commissure ... in ...
ceus and Dasypiis arc almost Monotreme-like."
4 Sec Wilson and Hill. Ouart* J. Mir.r. art. ^™v   iaaa  «  AW